1. Field of the Invention
The present invention relates to a thermostatic element of the type comprising an outer metal cup of elongated shape, containing a material that is notably expandable and contractile according to the direction of variation of its temperature, and a piston that can move relative to the cup in the longitudinal direction of the latter and is coupled to the expandable and contractile material so as to move in opposite directions depending on whether the material expands or contracts. The invention also relates to a thermostatic cartridge and faucet, fitted with such an element.
2. Brief Description of the Related Art
Such thermostatic elements are used in particular in the field of the temperature adjustment of a fluid originating from the mixing of two streams of fluid at different temperatures, the relative movement of the piston and of the cup being applied to modify the proportion of the mixing of the two streams of fluid. This is particularly the case in mixer faucet cartridges and in mixer faucets.
For a large number of applications in this field, it is necessary for the response of the thermostatic element to be extremely rapid, that is to say that the change of temperature of the environment in which the cup is formed very quickly causes a corresponding movement of the piston. This is particularly the case for thermostatic elements immersed in a stream of water supplying a sanitary installation, an application for which, an ideal temperature being selected, a drop in temperature of only three or four degrees is very unpleasant, and an increase of a few degrees may cause scalds.
The thermostatic elements conventionally used in this type of application comprise, for example in accordance with FIGS. 1 and 2, a metal cup 1 having a tubular unsupported portion 11 having a generally cylindrical shape with a circular base and a longitudinal axis X-X. A bottom end 12 closes this portion 11 while the opposite end spreads out to connect to a collar 13. A sheath 2, having a shape of revolution with a central channel 21, comprises a base 22 housed in the collar of the cup so that, except for the base 22, the sheath 2 extends outside the cup in the direction opposite to the cylindrical portion 11 of the latter, and coaxially. The collar 13 is swaged around the base 22.
The tubular portion 11 of the cup is filled with a mass of material that is extremely expandable and contractile according to the temperature variations, particularly around a functional temperature, here a mass of wax 3. The base 22 of the sheath comprises in its face that is opposite to this mass of wax, an annular housing 23 in which is anchored the periphery of a diaphragm 4 that is disk-shaped and elastically deformable, closing off the central channel 21 of the sheath on the side of the cup 1. Inside the channel 21 of the sheath, a piston 5 is housed that is subjected to the movements of the central region of the diaphragm by means of a pad 7 made of deformable elastomer in contact against the surface of the diaphragm opposite to the wax and a shim 8 made of polymer such as PTFE inserted between the pad and the piston and adjusted in the channel 21 to prevent the elastomer of the pad creeping around the piston. The end of the piston 5 opposite to the diaphragm 4 protrudes more or less from the sheath depending on the volume occupied by the wax, therefore on the temperature of the latter.
The general design of these thermostatic elements is well-suited to the use of a wax whose expansion coefficient is extremely high relative to that of the common fluids (approximately 10 to 20 times greater) and therefore capable of causing a very ample movement of the piston. Unfortunately, these waxes have a very low heat conductibility (approximately 1000 times less than that of copper), and therefore the temperature of the entire mass of wax only imperfectly and with a long delay reflects that of the fluid in which the cup is immersed. For this reason, the wax is usually “filled” with a powder made of a material having a good heat conductibility, for example a copper powder of appropriate particle size. For simplification, in the following description, the filled materials and the unfilled materials and the single-component waxes will all be called “wax”. However, all these artifices are insufficient to obtain a rapid-response thermostatic element that can be used without particular precaution in a sanitary installation.
To remedy this disadvantage in particular, it has been proposed, particularly in EP-A-0 153 555, to fit, inside the cup of the thermostatic element, a metal insert in contact with the inner face of the cup, as shown in FIGS. 1 and 2 in which this insert has reference number 6. This insert has, in cross section as in FIG. 2, a cross-shaped solid section whose four branches extend from the central zone of the thermostatic element to the tubular wall 11 of the cup 1. Along the axis X-X, the insert 6 extends over almost the whole length of the cup 1, its end opposite to the piston 5 being in contact with the bottom wall 12 of the cup. The insert 6 therefore divides the majority of the inner volume of the cup 1 into four distinct blind cavities 14 distributed about the axis X-X and all opening onto the diaphragm 4. The majority of the wax 3 is stored in these cavities, the rest of the wax being axially interposed between the diaphragm and the outlets of the cavities. In this manner, the heat of the tubular wall 11 and bottom wall 12 of the cup 1 is transmitted more rapidly to the inner metal insert 6 than to the wax 3, the latter then being heated by the insert in addition to its being heated by the walls of the cup.
The response time of this thermostatic element is therefore markedly improved. On the other hand, during the cooling phases of this thermostatic element, during which the wax 3 contracts, the wax may have difficulties in re-entering the cavities 14 of the cup 1 under the action of the piston 5 during retraction, because of the small cross section of the outlets of these cavities, through which the wax must withdraw toward the bottom wall 12 of the cup, leaving sufficient space for the retraction of the piston. These wax flow difficulties at the outlets of the cavities 14 are all the more marked when these cavities have a considerable depth, as is the case for a cup that is called “long”, that is to say a cup of the type of FIGS. 1 and 2, whose axial dimension is substantially greater than its diameter in order to increase the surface area of heat exchange between the wax and the inner face of the cup. In practice, the potential causes of these wax flow difficulties are particularly linked:                to a lack of homogeneity of the wax 3, in particular when the latter is filled with a heat-conducting powder, as mentioned above, since, after many cycles of expansion and contraction of this wax, the proportion of the conducting powder tends to increase in certain zones, which has the effect of locally increasing the viscosity of the filled wax, and/or        to dimensional differences of the cross sections of the various cavities 14, resulting from the design of the cup 1 and/or of the metal insert 6, and/or resulting from inaccuracies of assembly of this insert during the manufacture of the thermostatic element.        
Whatever is the cause thereof, the difficulty of flow of the wax 3 in one of the cavities 14 accentuates the hysteresis of the behaviors of the thermostatic element when it is heated and cooled since the piston 5 is slowed, and even stopped during its retraction for a temperature of actuation of the thermostatic element that is higher than that at which the piston should in principle stop its retraction travel, which results in decalibrating the thermostatic element.